Recommended Version for Mac OS X 10.5.8
General hardware age isn't the big problem here, and that's what the other commenters were talking about. Their advice is great if you have an x86 system of that vintage (ex, Pentium 4), but you don't. You have a PowerPC machine. That's problematic as the official OS X Blender builds are not compiled for PPC and haven't been for quite some time. Since 2.63, to be exact. You can find it here: http://download.blender.org/release/Blender2.63/

Download the file blender-2.63a-release-OSX_10.5_ppc.zip and install as usual. It should be your best bet.